University of Luxembourg

We address the problem of estimating the drift parameter in a system of N interacting particles driven by additive fractional Brownian motion of Hurst index H ≥ 1/2. Considering continuous observation of the interacting particles over a fixed interval [0, T ], we examine the asymptotic regime as N → ∞. Our main tool is a random variable reminiscent of the least squares estimator but unobservable due to its reliance on the Skorohod integral. We demonstrate that this object is consistent and asymptotically normal by establishing a quantitative propagation of chaos for Malliavin derivatives, which holds for any H ∈ (0, 1). Leveraging a connection between the divergence integral and the Young integral, we construct computable estimators of the drift parameter. These estimators are shown to be consistent and asymptotically Gaussian. Finally, a numerical study highlights the strong performance of the proposed estimators.

The 5,6,7,8-tetrahydrobiopterin (H 4 B) cofactor is essential for several enzymatic functions, including phenylalanine-4-hydroxylase (PAH, EC 1.14.16.1). However, H 4 B is highly unstable and spontaneously oxidizes in aqueous solution at room temperature, which complicates its measurement in clinical and research settings and has led to its exclusion from many routine analytical protocols. This study addresses the need for a quantitative description of H 4 B autoxidation and for practical strategies to mitigate analytical artefacts in solution and in urine. A kinetic study using flow injection analysis–tandem mass spectrometry (FIA–MS/MS) was carried out to monitor H 4 B signal decay under different media and antioxidant compositions, and the data were interpreted within a pseudo-first-order kinetic framework. Autoxidation was fastest in aqueous solvent and slower in urine, with rate constants further modulated by the presence of ascorbic acid (H 2 A) and glutathione (GSH). Equimolar H 2 A and GSH, and excess GSH, significantly slowed H 4 B decay but did not fully suppress oxidation, indicating partial stabilization rather than complete protection. Cyclic voltammetry and thermodynamic analysis provided complementary information on redox potentials, intermediates and thermodynamic affinity, linking changes in reaction rate to the underlying redox equilibria. The combined kinetic, electrochemical and thermodynamic results show that matrix composition and antioxidant content critically influence H 4 B stability in solution and in urine. While antioxidants such as H 2 A and GSH attenuate spontaneous oxidation, careful control of sample handling, timing and storage remains essential. The mechanistic insight obtained her e supports the rational design of stabilization and preparation protocols for future LC–MS/MS assays targeting H 4 B in bioanalytical and diagnostic applications. • H4B autoxidation kinetics were mapped in solution and urine. • FIA-MS/MS, voltammetry and modelling give a unified redox picture. • Antioxidants GSH and ascorbate slow H4B decay without full stabilisation. • Urine matrix partially stabilises H4B and modulates antioxidant effects. • Practical guidelines are proposed for handling H4B in bioanalytical assays.

The main goal of this work involves conducting numerical simulations to determine the vibrational behavior of composite shell structures. The introduction of doubly-curved laminated shells plays a pivotal role in diverse engineering applications, offering an expanded design space and the potential to enhance mechanical performance. However, the analysis of this kind of structures poses significant challenges due to the need to account for the curvature of the shell mid-surface. Moreover, the aspect ratio strongly influences the structural response in terms of free vibrations, also affecting the accuracy of the numerical solution. To address these complexities, this work employs the Carrera’s unified formulation, a well-established methodology for evaluating composites structural behavior which allows to set the expansion order of through-the-thickness polynomials as a free parameter of the simulation. In this context, the governing equations of the problem are derived through pure displacement and mixed formulations, within a finite element framework, ensuring a robust and adaptable analysis approach. This study extends the application of the unified formulation to complex shell geometries featuring non-uniform curvatures along the mid-surface principal directions. Free vibration analyses are performed to determine fundamental frequencies and mode shapes, with results benchmarked against 3D models from Abaqus and classical theoretical predictions. The comparison demonstrates the effectiveness of the proposed approach, establishing its efficacy for advanced structural investigation of fiber-reinforced shells with spatially varying curvatures, regardless of the considered slenderness ratio. • Carrera’s unified formulation is adopted for the modal analysis of curved shells. • Governing equations are derived using displacement-based and mixed theories. • Numerical results are validated against three-dimensional Abaqus reference models. • Lower-order models show accurate results for thin shells at reduced numerical cost. • Layer-wise mixed models achieve top accuracy.

In this study, we propose and validate a simple agent-based model to study cell-cell interactions and cell migration during in vitro scratch assays in the context of cutaneous fibrosis (keloid). For model parametrization, we collect data from in vitro experiments performed with healthy or keloid fibroblasts treated (or not) with type 1 or 2 macrophages secretome to mimic specific in vivo environments. All experiments were performed with mitomycin to inhibit cell proliferation, and subsequently isolate the sole contribution of migration to wound filling over time. The scratch assays are modeled within the cellular Potts model framework. The calibration process, via Levenberg-Maquart algorithm, gives a mean error of 4.53 ± 0.77% across the four modalities (healthy, control, M1 and M2 secretum) and the evaluation dataset gives a mean error of 10.55 ± 0.77%. With the help of this model, we test whether the hypothesis of contact inhibition of locomotion (CIL) can explain the movement of keloid fibroblasts. The simulation results and their comparison with the experimental data suggest that CIL might not characterize the movement of keloid fibroblasts, which is in contrast to the importance of CIL for the movement of healthy fibroblasts.

The role of geothermal energy in the energy transition is rapidly gaining recognition. Its storage technologies provide flexible solutions to address the intermittent nature of other renewable resources. Abandoned mines and other underground caverns have recently attracted particular attention as valuable geothermal reservoirs.In this context, a pilot project was recently initiated in the flooded abandoned underground slate exploitation area of Martelange (Belgium & Luxembourg). This area, with its long industrial history, represents a significant opportunity for future geothermal applications. The large volume of water stored within these underground caverns, in combination with heat-pumps, is sufficient to provide heating and cooling for a quite large number of buildings.Where natural regeneration, depending on groundwater flows and the thermal conductivity of the surrounding soil, proves insufficient and where water temperature changes exceed 5 °C, active regeneration may be implemented. This involves the installation of (un)glazed solar thermal absorbers to collect low-cost solar heat during summer and cold during winter, transforming the system into a form of solar district heating. In such a scenario, the flooded slate caverns would function as a seasonal energy storage body.The objective of the project is to develop a modern, highly energy-efficient urbanized area with a commercial zone, based on a low-temperature urban heat and cold network fed by heat-pumps. Achieving this requires a detailed assessment of the local geological and hydrogeological conditions, especially the structure and connections of the slate caverns, to ensure optimal and sustainable use of the available water resources. The first results from this study will be presented.

Design-flood estimation for small fluvial catchments in the Benelux plus Germany region relies predominantly on simplified event-based methods, reflecting limited data availability and the absence of consistent regional guidelines. While such approaches are widely applied in practice, key modelling choices are often unguided, leading to large uncertainty and inconsistent protection levels across regions. This study investigates how basin similarity, catchment storage, and geology can be used to improve the robustness of event-based flood estimation in data-fragmented settings. This study integrates high-resolution observations from a network of experimental and operational basins with hydro-meteorological records. Event-scale hydrological signatures relevant to event-based modelling (time to peak, rising-limb characteristics, peak discharge, and runoff coefficients) are derived and analysed across simple antecedent moisture conditions (AMC) classes. Basin similarity is assessed using a combination of physical descriptors and observed event responses, and basins are grouped into response classes using clustering techniques. Leave-one-basin-out testing is applied to evaluate the transferability of response characteristics within and across classes. A parsimonious event-based rainfall–runoff model, representative of methods commonly used in design-flood studies, is then applied. Model parameters are constrained using empirical ranges derived from observed events. Model experiments systematically vary response-time formulations, AMC assumptions, and parameter sources (regional data, proxy basin, or class-based values) to quantify their influence on simulated peak discharges. Sensitivity and robustness are evaluated across storage and geology classes. Results show that uncertainty in event-based design floods is dominated by response-time and AMC assumptions, and that similarity-based parameter transfer can reduce uncertainty in storage-controlled catchments but performs poorly where storage contrasts are large or data are sparse. The findings provide empirical guidance on when event-based regionalization is best suited and highlight structural limitations of current practice. This work supports the development of more consistent, evidence-based design-flood guidelines for small basins in the Benelux region.

Stable isotopes of oxygen and hydrogen are a powerful multipurpose tool widely used across multiple disciplines in Earth and Planetary sciences. In hydrology, δ18O and δ2H in the water molecule are commonly used in stream water source apportionment and transit time analyses. In paleoclimate research, ice core water isotope records are used as a temperature proxy, documenting past climate variability over hundreds of thousands of years. Oxygen and hydrogen isotopes are also versatile fingerprints for retracing the formation of planets and other celestial bodies.These examples should not obscure the fact that many unknowns and uncertainties remain inherent to the use of stable isotopes of O and H as tracers and fingerprints of processes in terrestrial and extra-terrestrial environments. To this date, only a few experimental studies have investigated water ice sublimation rates and the effect of isotopic fractionation processes – notably on water ice under lunar environmental conditions.Here we present results from a combined experimental and modelling approach. With an instrumental set-up developed at LIST, we simulate the sublimation of water ice under extreme environmental conditions (very high vacuum and/or very low temperatures) with the goal of exploring O-H isotopic fractionation processes in both (extreme) terrestrial and extraterrestrial environments. An understanding of these processes is necessary for interpreting the isotope signatures of water in planetary exploration missions, such as ESA’s PROSPECT project for lunar exploration, and in terrestrial hydrology of cold regions.The current experimental setup consists of a sublimation chamber capable of operating at pressures down to 10⁻⁶ Pa and temperatures as low as 110 K, with high stability and control over sublimation conditions. The system can simulate controlled environments for the phase transition of water (ice-vapor), isotopic fractionation, and the movement of water vapor across different phases of the experimental run. This includes transferring gas to a series of parallel cold traps, analyzing isotopic content using laser spectroscopy.We have developed a stochastic lagrangian numerical model to verify the existing theories of phase transition, diffusion, and O-H isotopic fractionation based on the Langevin equation. The model allows for sublimation, diffusive transport, and condensation of water and its isotopes through an isothermal domain representing the volume of the experimental prototype. Lagrangian models are highly adaptive for handling complex boundary conditions and well-suited for solving fluid mechanics problems with various types of particles.A sensitivity analysis of the model using different sublimation temperatures shows consistent results with our experimental data. Results obtained from the dual isotope analysis (δ¹⁸O and δ²H) of ice samples obtained from Greenland Summit Precipitation (GRESP) and Antarctica snow show trends consistent with theoretical predictions and meteoric water line, suggesting that the setup is operating reliably. Observed deviations in the isotopic compositions indicate influences from environmental variables such as humidity, pointing towards the need for tighter control and validation. Our experimental set-up lays a foundation for further investigations into the problems of fast diffusion, non-equilibrium thermodynamics, and the isotopic signature of water.